1. Field of the Invention
The present invention relates to a degassing method for an electrode paste.
2. Description of Related Art
Lithium ion secondary batteries are nonaqueous electrolyte secondary batteries that are chargeable-rechargeable by lithium ions in a nonaqueous electrolyte moving between positive electrodes and negative electrodes that store and discharge lithium ions. Large-capacity lithium ion secondary batteries have recently been installed in electric vehicles (EV) and plug-in hybrid vehicles (PHV).
Each of the negative electrodes and the positive electrodes of the above lithium ion secondary batteries has a layered structure in which an active material layer is formed on a current collector. The active material layer is formed by applying and drying a positive electrode paste or a negative electrode paste (referred to as an “electrode paste”, hereinafter) containing a solid content, such as an active material, a bonder, and a thickener, and a dispersion medium (solvent) onto the current collector, and thereafter, is press-processed.
Generally, the above electrode paste is produced by mixing the solid content, such as an active material, a bonder, and a thickener, and subsequently, kneading an obtained powder mixture and a dispersion medium. At this time, bubbles are usually contained in the kneaded electrode past; therefore, if this pates is applied onto the current collector as it is, no active material and others is applied to portions of the current collector in contact with the bubbles, which causes so-called “transparency defects”. Hence, a degassing process to eliminate bubbles in the electrode paste is carried out after the kneading process and before the application process.
Meanwhile, in the electrode paste, in order to reduce time required in a drying process for the sake of enhancement of productivity, a higher solid content ratio has been promoted by decreasing a mixture ratio of a dispersion medium. As the solid content ratio of the electrode paste becomes more increased, viscosity thereof becomes more increased, so that it becomes more difficult to release bubbles. In particular, the solid content ratio of the electrode paste of 50 mass % or more makes it difficult to perform degassing.
Japanese Patent Application Publication No. 2014-017064 discloses a technique in which while an electrode paste after being kneaded is introduced into a degassing tank to be formed into a film form, bubbles in the electrode paste is removed by decompressing the degassing tank. By setting a degree of vacuum to be approximately −90 kPa, degassing of the electrode paste having a solid content ratio as high as approximately 50 mass % becomes feasible. In the present specification, the degree of vacuum (degree of decompression) is represented in terms of gauge pressure.
The present inventors have found the following problems in the degassing method for an electrode paste described in JP 2014-017064 A. In the case of further increasing the solid content ratio of the electrode paste to be 55 mass % or more, if the degassing processing at a degree of vacuum of approximately −90 kPa is carried out, bubbles that are generated in the electrode paste by decompressing still remain, which might cause a large amount of transparency defects in the application process. Further increase in degree of vacuum reduces bubbles, but the electrode paste becomes likely to be dried in the degassing process because of the higher solid content ratio; consequently, striation defects are likely to occur during the application process.